1. Technical Field
The present invention relates to an image reading apparatus.
2. Related Art
There are image reading apparatuses, such as scanner apparatuses, in which shading correction is performed ahead of reading of an original document, in order to prevent unevenness in the read image due to non-uniformity in the light amount of a light source lamp, variations in the sensitivity of image reading elements, and the like (for example, see JP-A-2006-303710). White reference data, black reference data, and the like that have been quantized are used in shading correction.
As the number of gradations increases, quantized data can be expressed more finely and with higher accuracy. Consequently, shading correction can be performed with high accuracy by using white reference data or black reference data having a high number of gradations.
However, in an image reading apparatus with a narrow memory band, since data having a high number of gradations (high number of bits) cannot be rapidly processed, there is a case when shading correction is performed by decreasing the number of gradations of white reference data, black reference data, and the like to below the number of gradations of a read image. In particular, in a case of decreasing the number of gradations (number of bits) of black reference data, quantization errors may increase as data of lower order bits is insufficient, and an image after a shading process may be deteriorated.